With the growth of optical communications systems, the need for various multi-port optical devices has arisen. Such devices generally facilitate the implementation of new technology which increases the telecommunications capacity. For example, in effecting wavelength division multiplexed systems, three-port multiplexers for connecting sources with different wavelengths to a communication line, such as an optical fiber, are required. Additionally, the development of bidirectional transmission lines requires multi-port directional couplers which permit a terminal to receive a portion of a light signal while allowing the bulk of the signal to be transmitted to other terminals. More recently, all-optical, long distance communications systems have been proposed where fiber loss is periodically compensated by optical gain, such as from Raman or erbium amplifiers. Specifically this means that, in silica-based optical fiber, radiation of wavelength .lambda..sub.s can be amplified by means of pump radiation that is down-shifted in wavelength from .lambda..sub.s by a amount corresponding to shifts in wave number by about 100 to 600 cm.sup.-1. In such systems, polarization insensitive, wavelength dependent directional couplers are required for injecting the down-shifted Raman pump radiation.
The traditional approach of combining individual bulk components such as mirrors, lenses, prisms, and the like are limited by their poor insertion loss, large size, weight, and high cost. In attempts to overcome these difficulties, various proposals have been made for providing three and four port devices, such as couplers, switches, and demultiplexers/multiplexers, which are less costly and more reliable and efficient than prior art devices. For example, U.S. Pat. No. 4,213,677 discloses the use of a beam splitter between two graded index lenses. An optical signal from a fiber coupled to one surface of a lens is partially reflected from the beam splitter back to a fiber coupled to the same surface and also partially transmitted to a fiber on the opposite surface of another lens. By controlling the reflectivity of the beam splitter, the splitting ratio of the incoming light between two output ports may be adjusted. Replacing the beam splitter with an interference filter, consisting of a number of thin film layers, converts the the optical device into a wavelength demultiplexer or multiplexer. While such devices perform acceptably, once the transmissive characteristics of the filter or splitter have been set, the optical functionality and properties of the devices are immutable. Moreover, the cost associated with the fabrication of an interference filter is relatively high due to the number of layers required to achieved the desired transmission property.
Exemplary of the optical devices affording wavelength selective coupling is U.S. Pat. No. 4,768,849. There, an optical resonant cavity comprising two parallel dielectric mirrors permits resonance in a selected band of channels. Optical signals from a main trunk are coupled to one of the mirrors at an end face of the resonant cavity and, moreover, are coupled from the one mirror to an output port of the main trunk by evanescent coupling. Those signals to be coupled are selected according to the resonant condition of the cavity. However, because coupling between input and output ports in the main trunk is accomplished by evanescent coupling, it is necessary to employ tapered waveguides within the coupling region of interest or non-tapered waveguides suspended in air, which cause significant fabrication difficulties.
It is therefore an object of the invention to provide an optical device which is economical and affords efficient coupling among transmission elements as well as affords ease of fabrication. It is a further object of the invention to provide such a device where the functionality may be conveniently and dynamically altered to suit a particular need. Finally, it is an object of the invention to provide an optical device which is substantially insensitive to polarization.